1. Field of the Invention
The present invention generally relates to measurement of specific absorption rate (SAR), and more particularly, to an SAR measuring apparatus capable of quick and accurate measurement of a specific absorption rate.
2. Description of the Related Art
Specific absorption rate (SAR) is an index used to estimate an electric power level absorbed in a human body when a cellular phone is operated near the human body. SAR level is expressed by
                    SAR        =                              σ            ⁢                                                          E                                            2                                ρ                                    (        1        )            where σ is electric conductivity [S/m] of a medium, ρ is density [kg/m3] of the medium, and E is electric field [V/m]. See, for example, Thomas Schmid, Oliver Egger, and Niels Kuster, “Automated E-Field Scanning System for Dosimetric Assessment”, IEEE Trans. MTT-44, No. 1, pp. 105-113, January 1996.
Conventionally, when measuring specific absorption rate (SAR) values, electric field E generated in the medium is detected using a short dipole, and the detected electric field is converted to a specific absorption rate using Equation (1).
FIG. 1 is a schematic diagram illustrating a conventional specific absorption rate (SAR) measuring system. A phantom 11A, which is a human body model simulated using a liquid material having an electric properties similar to those of human tissues, is defined in a vessel 12. On the bottom of the vessel 12 is provided an electromagnetic wave radiating device 18, such as a cellular phone, to be observed to measure the influence on the human body. An electric field probe 13 is inserted and scanned in the phantom 11A by a positioning equipment 14 in the three-dimensional directions (x, y, and z directions) to detect the electric field generated in the phantom 11A. Based on the electric field measurement, specific absorption rate values are computed. The probe 13 is connected to an electric field measuring apparatus 16 via a signal transmission cable 15. The electric field detecting apparatus 16 determines the electric field levels based on the signal supplied from the probe 13 via the cable 15. A processor 17 controls the overall operations of the SAR measurement, as well as carrying out SAR computation and data analysis.
In the probe scan to measure the electric field in the phantom 11A, the electric field probe 13 is first driven in a two-dimensional manner along the bottom of the vessel 12 to acquire a two-dimensional SAR distribution. This scan is called area scan. A high SAR area is found by the area scan, and then, a three-dimensional scan is performed for the high SAR area and its vicinity to acquire more detailed (three-dimensional) information about the SAR distribution. This scan is called zoom scan.
FIG. 2 is a schematic diagram illustrating another conventional SAR measuring system, in which a solid phantom 11B is used. The electric field probe 13 is movable in the z direction, or alternatively, multiple probes are placed in the phantom 11B at different depths. The electromagnetic wave radiating device 18 is moved in the two-dimensional directions (x and y directions) by the scanning mechanism 19, while the electric field probe 13 is moved in the z direction. Using this arrangement, the electric field generated in the phantom 11B is measured, and SAR distribution is determined from the electric field measurement. The other operations are the same as those shown in FIG. 1.
In this manner, three-dimensional measurement of an electric field is performed by moving the electric field probe 13 in the three-dimensional directions in the phantom 11A in the example shown in FIG. 1, and by moving both the electric field probe 13 and the electromagnetic wave radiating device 18 to detect the three-dimensional distribution of the electric field in the phantom 11B in the example shown in FIG. 2. In either case, a specific absorption rate is obtained from the electric field measurement result.
However, the above-described measurement method has a problem in that two-step measurement for area scan and zoom scan has to be performed in order to obtain detailed information about the SAR distribution. This method requires time to complete the measurement.
To solve this problem, it is proposed to perform two-dimensional area scan (in the x-y plane) only and to calculate the electric field of the remaining dimension (in the z direction) based on the area scan measurement result using Equation (2), for the purpose of reducing measurement time.SAR(x,y,z)=SAR(x,y,zd)S(z,zd)  (2)See, for example, M. Y. Kanda, M. Ballen, M. G. Douglas, A. V. Gessner and C. K. Chou, “Fast SAR determination of gram-averaged SAR from 2-D coarse scans”, Abstract Book of the Bioelectromagnetics Society 25th Annual Meeting, Jun. 22-27, 2003; and M. G. Douglas, M. Y. Kanda and C. K. Chou, “Post-processing errors in peak spatial average SAR measurements of wireless handsets”, Abstract Book of the Bioelectromagnetics Society 25th Annual Meeting, Jun. 22-27, 2003.
In Equation (2), SAR(x, y, z) is an SAR estimate value at three-dimensional coordinates (x, y, z), and SAR(x, y, zd) is an SAR measurement result at z=zd. S(z, zd) is a function with respect to the depth direction (the z direction), By appropriately selecting function S(z, zd), a three-dimensional SAR distribution can be determined from a two-dimensional measurement.
Another known estimation method is to determine a three-dimensional SAR distribution using an elliptic function, based only on two-dimensional SAR measurements and one linear SAR measurements in the depth direction. See, for example, O. Merckel, J.-Ch. Bolomey, G. Fleury, “Extension of the parametric rapid SAR measurement to the SAM phantom”, Abstract Book of the 6th International Congress of the European Bioelectromagnetics Association, Nov. 13-15, 2003.
In the above-described method, the SAR distribution in the depth direction is estimated based on the two-dimensional SAR measurement result in order to reduce the measurement time. However, since the parameters in the estimating equations are determined empirically or approximately, it is difficult to estimate SAR distributions accurately under various situations. Due to this circumstance, the SAR measurement accuracy falls (producing uncertainty error), and accordingly, accurate measurement cannot be expected.